cpp-std-fwd

Forward declarations for most useful runtime classes of the C++ 17 standard library.

DISCLAIMER: This project is meant as a proof-of-concept for a proposal to standardize a forward declaration header for std. Using it is UB and should only be done to evaluate the proposal (see FAQ at the bottom).

Benchmarks

TL;DR: adding #include <stdfwd.hh> adds about 3 ms per translation unit.

Just compiling a single source including the specified file (compiler -std=c++17 -O0 -c /tmp/file.cc -o /tmp/file.o), best out of 10 compilations.

(system: intel i9-9900k 5 GHz, samsung 970 1TB pro nvme ssd, linux mint 19.1 tessa, kernel 4.15.0)

Windows benchmark done on Win10 with /GS /TP /W3 /Zc:wchar_t /Gm- /Od /Ob0 /Zc:inline /fp:precise /WX- /Zc:forScope /RTC1 /GR /Gd /MDd /std:c++17 /FC /EHsc /c.

Usage

Use forward declarations where possible in the header:

#include <stdfwd.hh>

stdfwd::string get_string();
stdfwd::vector<int> get_vector();
stdfwd::deque<int> get_deque();
stdfwd::list<int> get_list();
stdfwd::stack<int> get_stack();
stdfwd::forward_list<int> get_forward_list();
stdfwd::shared_ptr<int> get_shared_ptr();
stdfwd::unique_ptr<int> get_unique_ptr();
stdfwd::array<int, 3> get_array();
stdfwd::function<int()> get_function();
stdfwd::bitset<3> get_bitset();
stdfwd::pair<int, int> get_pair();
stdfwd::map<int, int> get_map();
stdfwd::set<int> get_set();
stdfwd::unordered_map<int, int> get_unordered_map();
stdfwd::unordered_set<int> get_unordered_set();
...

And in the .cc just #include <header> and define the functions using the normal std type (all declarations inside stdfwd are typedefs into std).

std:: forward declarations can also be used but won't have default template arguments (e.g. std::vector<int> does not work and std::vector<int, std::allocator<int>> or stdfwd::vector<int> must be used).

Adding support for map and unordered_map for custom data types:

#include <stdfwd.hh>

struct foo
{
    int v;

    bool operator==(foo const& r) const { return v == r.v; }
};

template <>
struct std::hash<foo>
{
    size_t operator()(foo const& f) const noexcept { return f.v; }
};

template <>
struct std::less<foo>
{
    bool operator()(foo const& a, foo const& b) const noexcept { return a.v < b.v; }
};

CMake

If the CMakeLists.txt is included, an interface library std-fwd is created:

# if set, force-includes "stdfwd.hh" in every translation unit
# this is optional and low-cost (see benchmarks)
# set(STDFWD_FORCE_INCLUDE ON CACHE BOOL "" FORCE)

add_subdirectory(path/to/cpp-std-fwd)

target_link_libraries(${PROJECT_NAME} PUBLIC std-fwd)

FAQ

• Why?

  Each non-trivial std header adds 50-200 ms compile time just by including them. #include <stdfwd.hh> is virtually free.

• Isn't adding declarations to std undefined behavior?

  Yes. This project is meant as a proof-of-concept for a proposal to standardize a forward declaration header for std.

• Why the namespace stdfwd?

  Some classes like std::vector have default template arguments which must only appear on a declaration once. If they appear on the definition then they cannot appear on the forward declaration. Thus, all forward declarations in std are without default arguments and stdfwd adds typedefs with the appropriate defaults.

• Why are some classes like std::integer_sequence missing?

  I omitted classes that are intended for compile-time programming because there seems to be little reason to forward declare them.

• What are the use cases?

  Many functions defined in a header but implemented elsewhere don't require complete definitions of their arguments or return types. std::vector<int> foo(); can be defined without including <vector> given forward declarations.

  Furthermore, many data types might want to specialize std::less or std::hash so that their users can use them inside std::map or std::unordered_map. Currently this requires including <functional> (one of the most expensive std headers to include) even if the data type itself uses nothing from that header. With a forward declaration, less and hash can be specialized without including the full header.

• Won't this be obsolete with modules?

  Yes and no. In C++20, std itself is not yet modularized. Additionally, it is not clear if all code bases can be immediately migrated to modules. This std forward header is a small and easy-to-implement non-intrusive change with big payoffs for many code bases.

• Why is iosfwd not used internally?

  This header should be as fast as possible. iosfwd includes more than forward declaration. It also fully defines the postypes at least in libstdc++.

Related Work

• https://blog.magnum.graphics/backstage/forward-declaring-stl-container-types/

TODO

• add support for libc++